It is known that the protection of an electrical installation from overvoltages can be achieved by using devices including at least one component for protection from overvoltages, for example one or more varistors and/or one or more spark gaps. For single phase installations, it is known to use a varistor connected between the phase and the neutral and a spark gap connected between the neutral and the ground. For three-phase installations, it is known to position varistors between the different phases and/or between each phase and the neutral and a spark gap between the neutral and the ground. For electrical installations operating under direct current, for example for photovoltaic generator installations, varistors and possibly spark gaps can be used.
In the event of failure of the protection component, these known devices include a disconnection system serving to isolate the protective component from the electrical installation as a safety measure. For example, in the case of varistors, it is known to provide thermal protection. The thermal protection or thermal disconnector can disconnect the varistor from the electrical installation to be protected in the event of excessive heating of the varistor, for example beyond 140° C. This excessive heating of the varistor is due to the increase of the leakage current—generally several tens of milliamperes—due to its aging, which is known as thermal runaway of the varistor.
The thermal disconnector often comprises (e.g., consists of) a low-temperature weld that keeps a conductive element in place to form a mobile contact through which the varistor is connected to the electrical installation, when the conductive element is elastically stressed towards the opening. The fusion of the weld results in the mobile contact moving under the effect of the elastic stress, which causes the disconnection of the varistor. Thermal disconnectors of this type are described in EP-A-0 716 493, EP-A-0 905 839, and EP-A-0 987 803, each of which is hereby incorporated by reference in its entirety.
These known devices which protect against overvoltages, and their thermal disconnector, can be faced with different restrictive situations during their use. The restrictive situations can depend, for example, on the type of electrical grid to which they are attached.
First, their thermal disconnector should have a sufficient interrupting capacity to effectively disconnect the protection component in case of thermal runaway. This constraint can be more delicate in the case of installations operating under direct current, given that there is no periodic passage at zero volts, as with alternating current. The alternating current contributes to the extension of the electric arc generated at the opening of the mobile contact.
The electrical circuit of the protective devices shall also be able to support the constraints resulting from electrical shocks, such as the lightning currents for which they are provided. These electric shocks can be surges with a significant amplitude (e.g., several thousand volts) and short duration (e.g., from a microsecond to a millisecond). These overvoltages, for example, can cause electrodynamic stresses and temperature increases that mechanically stress the different conductive pieces making up the protection device. Despite these mechanical stresses, the electrical circuit ensuring the connection of the protective component to the electrical installation should remain closed. In particular, the mechanical stresses should not cause the thermal disconnector to turn on via pulling out of the thermofusible braze. The ability of the device to meet this constraint can be verified by the applicable standards, for example, in installations supplied with low-voltage alternating current, in paragraph 7.6 (operating duty tests) of standard IEC 61643-1, 2nd ed., 2005-03 (hereafter noted IEC paragraph 7.6), or paragraph 37 (Surge testing) of standard UL 1449, 3rd ed., Sep. 29, 2006 (hereafter noted UL paragraph 37). For direct current installations such as photovoltaic generator installations, examples include paragraph 6.6 (Operating duty tests) of photovoltaic guide UTE C 61-740-51 dated June 2009 (hereafter UTE paragraph 6.6).
Moreover, the electric circuit of the protective device connecting the protective component to the electrical installation can be subject to very high currents under the nominal voltage of the electrical installation, for example in installations powered by the alternating voltage grid. This example occurs when the varistor of the protection device experiences a power outage by short circuit. In this case, the disconnection of the failing varistor is caused by a specific protection from short circuits such as a fuse or a circuit-breaker. Given the reaction time of this specific protection, the electric circuit of the protection device, including the thermal disconnector, should not cause any fire outbreak in that period of time, given the significance of the short circuit currents provided by the electrical power grid. The ability of the device to satisfy this constraint can be verified for installations powered with low-voltage alternating current, for example in paragraph 7.7.3 (Short circuit withstand) of standard IEC 61643-1, 2nd ed., 2005-03 (hereafter noted IEC paragraph 7.7.3).
The device for protection from overvoltages can also be capable of being powered by a surge related to an anomaly in the voltage of the power grid of the electrical installation, when a power outage caused by a short circuit of a varistor if there are at least two varistors serially connected between the lines of the power grid. In such a case, the varistor turns on and can pass a very high current given its low independence. The current is more or less the short circuit current that the power grid of the electrical installation can supply. Faced with such a situation, the protective device should not cause a fire to start.
The ability of the protective device to satisfy this constraint can be verified for installations supplied with low-voltage alternating current, for example in paragraph 39 (Current testing) of standard UL 1449, 3rd 3d., Sept. 29, 2006 (hereafter noted UL paragraph 39), or for photovoltaic generator installations, for example in paragraph 6.7.4 (End of life tests) from photovoltaic guide UTE C 61-740-51 dated June 2009 (hereafter noted UTE paragraph 6.7.4).
These protective devices should therefore, depending on the case, satisfy a number of constraints. One of the main constraints for the thermal disconnector can include reliably cutting the electric currents passing through, under circumstances that cause them to be turned on and to cut the electrical arc created between the mobile contact and the fixed contact(s) from which it separates. From this perspective, it is appropriate to have a sufficient and substantial interrupting capacity despite the frequently reduced bulk of these protective devices. This can be more strict in direct voltage installations given the fact that the extinction of the electric arcs is not made easier by the passage by zero volts of voltage as is the case for alternating current.